1. Field of the Invention
The present invention relates to an analog/digital (A/D) convertor and more particularly to an A/D convertor used in an video coder-decoder (CODEC) for the Integrated Services Digital Network (ISDN).
2. Background Art
There is an A/D convertor that converts an input analog signal such as video signal into an output digital signal made up of a specified number of bits. Also available are a video CODEC incorporating the A/D convertor and a wide-band ISDN that includes the video CODEC.
The wide-band ISDN requires an A/D convertor that rapidly converts video signals of wide frequency band into output digital signals of relatively few bits. Among the A/D convertors that meet this requirement is a so-called total parallel comparison type A/D convertor that instantly compares all the sampled input analog signals with all reference voltages. This type, however, requires 255 comparators (voltage comparison circuits), a significant amount of hardware, only to obtain 8-bit output digital signals.
To cope with this situation, what is generally called a pipeline type A/D convertor is proposed, which has comparators assigned one for each bit of the output digital signal and performs a pipeline processing in which the reference voltages given to the comparators are selected according to the comparison results of comparators for higher-order bits.
The pipeline type A/D convertor is introduced in the collection of IEEE papers 1987 "Custom Integrated Circuits Conference," page 694-697.
The inventors of this invention has found that the pipeline type A/D convertor mentioned above has the following drawbacks. Although the compare operation of each comparator and the precharging of the reference voltages can be completed in a relatively short period of time, the precharging of the reference voltages is performed only after the comparators for the high-order bits have produced comparison results and then the compare operation is initiated when the precharging operation is finished. In other words, the time taken by the precharging operation and the compare operation depends on the settling time of each sample-and-hold circuit, and determines the sampling speed of the A/D convertor. This puts constraints on high-speed operation of the A/D convertor, which in turn degrades the overall performance of the video CODEC incorporating the A/D convertor and also of the wide-band ISDN.